This invention relates to compositions employing electrically conductive polymeric materials which are particularly suitable for use in electrochromic systems, to processes for making those compositions and to electrochromic displays using those compositions.
Several reduction-oxidation (redox) active materials are very intensely colored in one redox state, but not in another. Such materials are called electrochromic. Ideally, electrochromic materials for display applications are very insoluble and can be coated on conductive surfaces. A preferred method for coating electrochromic materials onto electrode surfaces is by electrochemical deposition. Several problems have been encountered with electrochromic coatings, e.g. their lack of long term stability, probably due to poor adhesion to electrode surfaces, and their switching speed which diminishes as the coating is made thicker in order to improve the optical effects.
There has been considerable interest in the application of conductive polymers in electrochromic displays. The term "conductive polymer" refers to a class of polymeric materials that possesses electrical conductivity and which can sometimes be comparable to that of metals. Unlike metals, however, conductive polymers are not always conductive. They are usually in a conductive state only when at least partially oxidized. Reduced (i.e., neutral) conductive polymers usually have conductivities that are several orders of magnitude lower than their conductivities when oxidized.
Conducting polymers have distinctly desirable features for display applications. These include rapid response to an applied potential (i.e., high switching speed), durability, low average power consumption under repetitive potential cycling and an extremely low solubility that makes them ideal for coating on electrodes. Unfortunately, while some electrically conductive polymers exhibit electrochromic properties, a disadvantage in display applications is that in order to realize intense coloration, one has to use them in layers (coatings) of such thickness that their switching speed is adversely affected. Moreover when used in thick coatings they have poor durability and their average power consumption is high.
A need exists for electrochromic materials which can be immobilized on electrode surfaces for use in display systems. Moreover, there is a need for new electrochromic materials that have a rapid response, high durability and low average power consumption as well as intense coloration.